Fuel Pressure

[Taylorman]

I have a new fuel pressure problem. I am currently running a Demon 850 on a Victor intake on my 455. At wot im loosing fuel pressure. It starts dropping off at about 4500 rpms. Prior to this, i had a tunnel ram with dual edelbrock 600's. Ran this setup for 3 years and never had a fuel pressure problem.
I have the same fuel lines now that i had with my dual carbs. Im running -6x1/4 npt lines and fitting out the tanks into a Raycor fuel filter, to the fuel pump and into the carbs with a Aeromotive bypass pressure regulator which returns fuel to the 2nd inlet on the Raycor filter. I removed the regulator and and same thing happens, loss of fuel pressure at wot.
Is it possible that a single Demon requires more fuel than dual Edelbrocks? How should i troubleshoot? Im thinking its either fuel pump, filter too restrictive, or lines to small. What ya'll think?

[oldbuck40]

I have a new fuel pressure problem. I am currently running a Demon 850 on a Victor intake on my 455. At wot im loosing fuel pressure. It starts dropping off at about 4500 rpms. Prior to this, i had a tunnel ram with dual edelbrock 600's. Ran this setup for 3 years and never had a fuel pressure problem.
I have the same fuel lines now that i had with my dual carbs. Im running -6x1/4 npt lines and fitting out the tanks into a Raycor fuel filter, to the fuel pump and into the carbs with a Aeromotive bypass pressure regulator which returns fuel to the 2nd inlet on the Raycor filter. I removed the regulator and and same thing happens, loss of fuel pressure at wot.
Is it possible that a single Demon requires more fuel than dual Edelbrocks? How should i troubleshoot? Im thinking its either fuel pump, filter too restrictive, or lines to small. What ya'll think?hey Kevin i run mine at 7lbs and dont seem to have a problem,and no return line. whats your pressure at idle?

[Taylorman]

I set my regulator at 7psi. It stays at 7 up to about 4200 then above that it starts dropping.
I ran it without the regulator and it has 9 lbs but does that same thing at wot.
What size fittings you have coming out of your fuel tanks?

[oldbuck40]

I set my regulator at 7psi. It stays at 7 up to about 4200 then above that it starts dropping.
I ran it without the regulator and it has 9 lbs but does that same thing at wot.
What size fittings you have coming out of your fuel tanks?3/8 all the way from the tank to the 110gph blue elec,pump then up to a 110gph mechanical,,,my pressure drops some but doesnt have any effect on performance,not sure how much,cant see the gauge while driving. and b/4 you say anything about my 2 pumps,well lets just put it this way,me and that paddle dont get along :devil: if one goes out i can bypass either and still get home.

[SmokinLowriderSS]

I think I'd upsize the lines and put in dual paralell filters. Then, if you still have a pressure loss problem, then it might be the pump. Either it getting weak or it is insufficient for the aplication.
I'm at a loss to see how the single 4 would demand nore fuel than the 2X unless the single is a better "fit" to the aplication and is being drawn from harder then the 2 (adding up to a much larger single) was. Meaning,you might only have been pulling 800CFM though the 2x600's (1200cfm capability) and now pulling more fuel through the 850 doe to a better (higher) carb flow signal.

[Oldsquirt]

Kevin, you have a fuel supply problem and you haven't changed the filter? I would certainly do that BEFORE any redesigning of the system. The new intake/carb setup should not have increased fuel demand to the point where the old system is suddenly inadequate.
Verify the tanks are properly vented. Since you have saddle tanks, try running off each tank separately to see if the results are different.

[Taylorman]

Oldsquirt, i change the filter once a year at the beginning of the summer. Ive never had this problem. It just started when i put on the new carb and intake. Im gonna hook my fuel pump up directly to the fuel tank to see what happens.

[Kindsvater Flat]

I used to have the same problem. I only changed my filter once a year also. I now change it once a month during boating season.
What fuel pump do you have? Mine electric pumps were only on the boat for a year and they went bad. They would put out 6lbs at idle but at wot for 4-5 sec they would fall fast. New pump cured it.
Also I went from a pump that only put out 7psi to begin with. Now I'm running a 18psi pump with this crappy regulator.
http://www.schoutenranch.com/gallery/albums/userpics/10001/fit4.JPG

[OMEGA_BUBBLE_JET]

definitely check your lines in your tanks. I know with mine......the anti-siphon valves had been removed and replaced with a 1/8th inch vacuum hose for a dip tube :eek: .......no kidding. I used to run out of fuel on the top end and lean pop thru the carb. when I had my new tanks built a couple years ago I had him put in 1/2" stainless tubing with a 1/2" bung on top of the tank. I then put a bushing in it to bring it down to 3/8" and run 3/8" fuel line from the tanks to the carb. Absolutely zero supply problems now. I can turn 5400 rpm as long as I hold the go pedal down:)
Omega

[Stingray69]

Run -6 on anything hot.
Double check float levels.
Gage at carb inlet.
Check for vacuum leaks under carb and at intake faces.
Your sure that it's not just jetted lean at high RPM?

[Taylorman]

FLOAT LEVELS. Lets talk about that. Im going off of the fuel in the sight glasses. The fuel is about 3/4 to the top of the sight glasses. I set them on the trailer at idle. Is this correct? Fuel pressure gauge at the carb inlet. How do you check for vaccum leaks?

[Oldsquirt]

Fuel level is too high. Unless Barry Grant is different than the Holley, it should be right at the bottom of the glass. It's better to set them in the water unless you can accurately match, while on the trailer, the angle the boat sits in the water.
Vacuum leaks wont affect your fuel pressure, but can affect idle quality, and if serious enough, can affect higher rpm operation. Quick and dirty check is to spray suspected areas with a quick spritz of carb cleaner. If you get a rpm increase, you have found a leak. Dont spray so much that it gets pulled up and into the carb as that will give a false response.
Even though the filter was new this year, it only takes one bad tank of gas to clog it . Change it regardless.

[Stingray69]

SET FLOATS AS BOAT SITS IN THE WATER! Angle spacers will tilt carb back to level if severely tilted, this is fairly important.
I have only used the WD-40 method of spraying all gasket joints and ports/plugs with the little hose;stream at everything, with the motor idleing. Hit carb baseplate and ports/caps. Do it cold and hot. If motor bogs when you spray at something, there's the leak. Good to go around everything whenever you do an intake. A vacuum gage, while doing this, can help. Liquid fills hole temporary, vac changes.
And about your fuel pump; it was fine 'til you changed intake and carb.
So was my sump pump, when the water heater died. Power vent water heaters suck. To drain it I ran a hose to sump pump. Pump dies and I have to replace it before I can get old w h out of basement. Over $600 total. Maybe your fuel pump is starting to wave goodbye...